Many modern day electronic devices are often enclosed in what are referred to as surface mount packages, that is, packages generally intended to sit on or above a wiring board and having leads formed in such a way as to readily attach (e.g., by soldering) to contact regions on the surface of the wiring board, as opposed to being inserted into contact holes passing through the wiring board. By way of illustration and not intended to be limiting, FIG. 1A is a top view and FIG. 1B is a side view of high power light emitting diode (LED) 10 having surface mount package 12. LED 10 in package 12 has electrical leads or contacts 14–17 protruding laterally from sides 18–19 of LED package 12. Leads 14–15 of package 12 are formed downward and outward so that foot portions 20, 21 of leads 14, 15 are approximately parallel to base 22 of package 12. LED 10 conveniently emits light through lens 24 opposite to package base 22, but this is not essential. Persons of skill in the art will understand that although device 10 is identified herein as an LED, this is merely for convenience of description and not intended to be limiting. The problem described herein and the present invention apply to any type of electronic device having a generally surface mount lead configuration and substantially planar lower surface for contacting a heat sink.
With most surface mount packages the heat being generated by the internal electronic circuit or semiconductor chip is primarily extracted or dissipated through base 22 of package 12. Thus, it is important, especially with high power dissipation devices, that good thermal contact is made to base 22 of package 12. FIGS. 2A–B are partial cross-sectional views illustrating prior art arrangements for providing thermal contact to base 22 of package 12. In FIG. 2A, device 10 is surface mounted on wiring board 26 by, for example, soldering leads 14, 15 to electrical contact regions (not shown) on upper surface 25 of wiring board 26. Wiring board 26 is often referred to as a “printed circuit board” (PCB) or “printed wiring board” (PWB). For convenience, the abbreviation PWB is used herein. PWBs generally have a core of insulating material (e.g., plastic impregnated fiberglass) on which copper (or other highly conductive metal) foil “wires” have been formed, including the contact regions on surface 25 to which leads 14, 15 of device 10 are to be soldered. PWBs are well known in the art. For simplicity, the conductive metal leads and contact regions present on PWB 26 have been omitted in FIGS. 2A–B. However persons of skill in the art will understand that PWB 26 in FIGS. 2A–B (and PWB 42 in FIGS. 3A–B, 4) have such conductive leads and contacts in locations appropriate to the circuit being implemented and the location of the devices being placed thereon.
The insulating core of the PWB is generally a poor thermal conductor. For this reason, even though lower surface 27 of PWB 26 is in contact with upper surface 32 of heatsink 30, PWB 26 does not contribute greatly to heat disipation from device 10. Accordingly, it has been common in the prior art to provide metal insert regions 28 that act as thermal vias, underlying base 22 of package 12. These metal inserts or thermal vias (the terms are used interchangeably herein) reduce the thermal impedance between package base 22 and heatsink 30. It is common in the prior art to use a thermally conductive grease or adhesive between base 22 and thermal vias 28 and also between thermal vias 28 and heatsink 32. An adhesive provides the best heat conduction but prevents or greatly hinders replacement of defective LEDs and adds to the manufacturing cost of the assembly of FIG. 2A. Metal insert regions 28 are often formed by plating but other methods can also be used. A further disadvantage of the arrangement of FIG. 2A is that forming metal insert regions 28 is costly.
FIG. 2B is a cross-sectional view similar to FIG. 2A but showing a different arrangement used in the prior art for providing heat sinking of device 10. In FIG. 2B, heatsink 30′ is provided with pillar 33 which makes direct contact at surface 32′ with base 22 of device 10. While this arrangement generally provides good thermal contact to device 10, heatsink 30′ must be machine to have pillar(s) 33 in the correct location(s) to match hole(s) 29 in the location(s) of device(s) 10. This can be very costly since, in general, standard heatsinks cannot be used and must be custom machined for each PWB configuration. This is a significant disadvantage, especially where the PWB contains a large number of high power dissipation devices.
Accordingly, it is desirable to provide an improved and less costly means and method for heatsinking surface mount electronic devices. In addition, it is desirable to provide an arrangement and method wherein the devices are held in compression both with respect to their leads and the heat dissipation surface of the device package. Still further, it is desirable to provide a means and method for improved heat dissipation that is especially well adapted to PWBs having an array of high dissipation electronic devices. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.